text: It actually might depend on what you call 'interstellar' vs 'intergalactic'. [This paper](_URL_2_) from this year describes where all the baryons are in and around a typical galaxy. Unfortunately you have to scroll all the way through to Figure 11 for the key result, but it basically says: * ~10% of normal matter in a galaxy is in stars. * ~10-15% of normal matter is in the 'interstellar' medium between the stars in the disk of the galaxy. * The other ~75% of normal baryonic matter is in the 'circumgalactic' or 'intergalactic' medium of diffuse gas surrounding and in between galaxies.